The present invention relates to vehicle detectors which detect the passage or presence of a vehicle over a defined area of a roadway. In particular, the present invention relates to measurement frame segmentation in vehicle detectors as a means for shortening the detector output response time while maintaining detector sensitivity sufficient to detect small changes in inductance of the connected inductive sensor.
Inductive sensors are used for a wide variety of detection systems. For example, inductive sensors are used in systems which detect the presence of conductive or ferromagnetic articles within a specified area. Vehicle detectors are a common type of detection systems in which inductive sensors are used.
Vehicle detectors are used in traffic control systems to provide input data required by a controller to control signal lights. Vehicle detectors are connected to one or more inductive sensors and operate on the principle of an inductance change caused by the movement of a vehicle in the vicinity of the inductive sensor. The inductive sensor can take a number of different forms, but commonly is a wire loop which is buried in the roadway and which acts as an inductor.
The vehicle detector generally includes circuitry which operates in conjunction with the inductive sensor to measure changes in inductance and to provide output signals as a function of those inductance changes. The vehicle detector includes an oscillator circuit which produces a oscillator output signal having a frequency which is dependent on sensor inductance. The sensor inductance is in turn dependent on whether the inductive sensor is loaded by the presence of a vehicle. The sensor is driven as a part of a resonant circuit of the oscillator. The vehicle detector measures changes in inductance in the sensor by monitoring the frequency of the oscillator output signal.
Examples of vehicle detectors are shown, for example, in U.S. Pat. No. 3,943,339 (Koerner et al.) and in U.S. Pat. No. 3,989,932 (Koerner).
The duration of a measurement period required to detect a specific change in inductance is quite long when a small (e.g., 16 nanohenries) inductance change caused by a motorcycle or bicycle must be ascertained. Detection of automobiles, which cause larger inductance changes (e.g., greater than 3000 nanohenries on an inductive sensor in the form of a 3 turn, 6'.times.6' loop), may be accomplished with shorter measurement periods. In a detector that sequentially activates several inductive sensors, the response time of the detector to the presence of a vehicle over any one inductive sensor is determined by the summation of the time spent measuring the frequency of each of the inductive sensors. This becomes very important when vehicle speed is being measured. As the time spent measuring each inductive sensor increases, the ability to accurately estimate vehicle speed decreases. The ideal situation for speed measurement would be to spend a small amount of time measuring each inductive sensor regardless of the magnitude of the threshold change in inductance that is being measured.
In the past, vehicle detectors have typically utilized long measurement periods in order to ensure detection of small inductance changes. Prior art vehicle detectors are capable of measuring a wide range of inductance changes, but they are not capable of measuring small inductance changes while simultaneously utilizing short measurement periods. This is significant because the ability of the inductive sensor to measure vehicle speeds is a function of the measurement period length.